1. Right ventricular rabbit papillary muscles were arterially perfused with a mixture of Tyrode solution, bovine erythrocytes, dextran and albumin. In the recording chamber, they were surrounded by a H2O-saturated atmosphere of O2 and CO2 which served as an electrical insulator. 2. Conduction velocity and passive electrical properties were determined from intra- and extracellular potentials measured during excitation and during flow of subthreshold current. 3. The propagation of the action potential was linear along the muscle at a velocity of 55.6 cm/s. The extracellular wave-front voltage was 51.5 mV. 4. The following values for passive cable properties were obtained: (i) a ratio of extra- to intracellular longitudinal resistance of 1.2; (ii) an extracellular specific resistance (Ro) of 63 omega cm; (iii) an intracellular specific resistance (Ri) of 166 omega cm; (iv) a space constant lambda of 0.357 mm; (v) a membrane time constant tau of 2.57 ms. The space constant lambda* recalculated for zero extracellular resistance was 0.528 mm. 5. Arresting perfusion with drop of perfusion pressure was associated with an immediate increase of the extracellular longitudinal resistance by 35% and a decrease of conduction velocity by 13%. 6. The present results demonstrate the important contribution of the extracellular resistance to electrotonic interaction and propagation in densely packed myocardial tissue. Moreover, changes in perfusion pressure are associated with changes in extracellular resistance, probably as a consequence of changes in intravascular volume.